Does Objective-C guarantee the initialization of interface member data?

In an interface such as this:

@interface M开发者_Go百科yClass : NSObject
{
    bool PlainOldBool;
}

@end

... does PlainOldBool get auto-initialized to false, or is it necessary to use an init method to do that explicitly?

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Yes (unless your false is not 0). The default +alloc/+allocWithZone: method will automatically zero out all ivars.

From https://developer.apple.com/library/ios/documentation/Cocoa/Conceptual/CocoaFundamentals/CocoaObjects/CocoaObjects.html#//apple_ref/doc/uid/TP40002974-CH4-SW17

• It initializes all other instance variables to zero (or to the equivalent type for zero, such as nil, NULL, and 0.0).

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Also worth noting is that if you're doing Objective-C++, C++ objects that are ivars of Objective-C objects are not initalized : their constructors are not called by default.